1. Field of the Invention
The present invention relates to a lead-acid battery, particularly a carbon material for a lead-acid battery superior in high-efficiency charging characteristic.
2. Related Art Statement
A lead-acid battery as a secondary battery is relatively inexpensive and has stable properties; therefore, has been widely used as an electric source for automobiles and portable apparatuses, or as a back-up electric source for computers. Recently, a lead-acid battery has found new applications as a main electric source for electric cars, as an electric source for start-up of hybrid electric cars and simple hybrid cars, or for recovery of regenerated current. In these new applications, a lead-acid battery need to have, in particular, both of high output characteristic and high input characteristic.
Various studies have heretofore been made on the high output characteristic. With respect to the high input characteristic, however, there has been obtained no level which is superior to those of other battery systems.
The high input characteristic, i.e. high-efficiency charging characteristic of a lead-acid battery is greatly influenced by the characteristics of the lead sulfate present in the anode. With respect to the anode active material of a lead-acid battery, metallic lead emits electrons and is converted into lead sulfate in the discharge reaction; in the charging reaction, lead sulfate accepts electrons and is converted into metallic lead. The lead sulfate generated during discharge has neither ionic conductivity nor electron conductivity and is an insulating material. Further, the lead sulfate is very low in solubility into lead ion. Thus, lead sulfate is low in electron or ionic conductivity and moreover low in solubility into lead ion; therefore, the rate of reaction from lead sulfate into metallic lead is small, resulting in inferior high-efficiency charging characteristic.
As countermeasure therefor, improvements of charging characteristic have been tried, for example, by optimizing the amount of carbon added into an anode active material (JP-A-9-213336) or by allowing an anode active material to contain metallic tin (JP-A-5-89873).